This invention relates generally to the field of subsurface environmental monitoring of contaminated sites, and more specifically to the use of microsensors for in-situ real-time detection, long-term monitoring, and automated remediation of volatile contaminants (VCs), including volatile organic compounds (VOCs), in contaminated soil, aquifers, bodies of water, inside of monitoring wells, and around underground storage tanks or underground pipes, and to methods of characterizing subsurface volatile contaminants using in-situ sensors.
Volatile organic compounds (VOCs), also known as Non-Aqueous Phase Liquids (NAPLs), are the principal contaminants at many commercial and DOE sites. Some examples of VOCs include compounds such as aromatic hydrocarbons (e.g., benzene, toluene, xylenes); halogenated/chlorinated hydrocarbons (e.g., trichloroethylene (TCE), carbon tetrachloride); alcohols, ketones (e.g., acetone) and aliphatic hydrocarbons (e.g., hexane, octane). Other VCs of interest to groundwater protection include methyl tert-butyl ether (MTBE), other gasoline additives, toluene, and xylene (See 40 CFR 141.32 Primary Drinking Water Standards). Volatile contaminants can also include toxic chemicals, volatile pesticides, volatile fertilizers, buried volatile explosives, and organic compounds with low volatility. VC's can include gases or vapors other than volatile organic compounds, such as nitrogen oxide, nitrous oxide, carbon monoxide, carbon dioxide, hydrogen gas, and toxic gases, such as ammonia, chlorine, phosphonates, nerve gas (mustard, sarin, VX).
Tens of thousands of sites containing toxic chemical spills, leaking underground storage tanks, and chemical waste dumps require characterization and long-term monitoring (stewardship) to protect environmental resources (e.g., groundwater) and to determine when remedial measures are needed. Current methods are costly and time-intensive, and limitations in sampling and analytical techniques exist. For example, the Department of Energy (DOE) Savannah River Site requires manual collection of nearly 40,000 groundwater samples per year, which can cost between $100 to $1,000 per sample for off-site analysis (not including the cost of collecting the samples). Numerous commercial sites and applications, which include over two million underground storage tanks (e.g., at gas stations), also require monitoring to satisfy EPA requirements; as well as thousands of commercially contaminated sites that require characterization, monitoring, and/or remediation. Also, oil and natural gas fields currently take individual fluid samples manually from wells at a cost of nearly $250,000 per sample.
Another problem with current techniques is that the integrity of off-site analyses of contaminated samples can be compromised during sample collection, transport, and storage. The volatile compounds (VCs) may evaporate if the samples are exposed to the atmosphere when they are handled or stored. Measured concentrations using these ex-situ analysis methods can therefore be lower than actual in-situ concentrations.
The use of real-time sensors that can be placed in-situ would reduce the need for collecting manual samples and performing expensive off-site analyses. However, while technology exists to detect and analyze VOCs, very few systems are rugged enough to be deployed in-situ (e.g., in soil and water) while providing real-time, continuous, and reliable long-term monitoring. Many of these devices include sensitive electronic components (which can be damaged by lightening-induced ground currents) and require the flow of an inert carrier gas during operation (i.e., to carry the contaminant vapor up to a chemical analysis unit located on the surface), which may not be amenable to long-term in-situ monitoring and remediation applications.
Petrovend, Inc., of Hodgkins, Ill. (www.petrovend.com) manufactures a hydrocarbon vapor sensor probe that consists of a single chemiresistor sensor element (ADSISTOR™) mounted in a housing that has multiple openings through which vapors can freely flow. The probe's housing is not waterproof and, hence, cannot be placed in liquid-saturated soil, water, or other liquids contaminated with VOCs.
Geoprobe Systems, Inc. of Salina, Kans. (www.geoprobesystems.com) manufactures a sampling device called a Geoprobe Membrane Interface Probe, is a cone penetrometer that has a gas permeable membrane covering an opening in the pointed tip (i.e., cone). The membrane selectively allows VOC gases in soil to diffuse through the membrane into the interior of the penetrometer. A gas supply & return tube located inside the penetrometer interfaces with the gas permeable membrane. A carrier gas is pumped through this tube to sweep up and carry VOCs (that have diffused through the membrane) up to a detector/analysis unit located on the surface. The Geoprobe Membrane Interface Probe performs in-situ sampling by pumping and/or drawing the diffused VOC vapor up to the surface via a carrier gas. However, the Geoprobe Membrane Interface Probe does not perform in-situ sensing or analysis, because sensing is performed by a remote detector/analysis unit located on the surface.
Many microsensor systems, such as portable hand-held spectrometers, gas chromatographs, catalytic bead sensors, and metal-oxide-semiconductor (MOS) sensors, utilize complex and sensitive electronic components, can use high temperature elements, and can require the flow of a carrier gas during operation to draw subsurface vapors up to the surface and into a chamber for sensing, none of which may be amenable to long-term in-situ monitoring applications. For example, the “Cyranose” hand-held electronic nose manufactured by Cyrano Sciences, Inc. (www.cyranosciences.com) is a chemiresistor-based microsensor that uses a pump to draw vapors into a sensing chamber. The Cyranose sensor unit, however, cannot be submerged in water for in-situ monitoring because the housing is not waterproof.
Polymer-based vapor absorbtion type sensors are attractive choices for use in VC monitoring devices. Examples of these sensors include conductometric sensors such as chemiresistors, surface or thickness-shear mode acoustic wave (SAW) mass sensors, flexural plate wave mass sensors, and MEMS microcantilever mass sensors. Chemiresistors are a particularly simple type of chemical sensor whose electrical resistance changes in the presence of certain chemical vapors. Chemiresistors are easy to fabricate using well-known semiconductor fabrication techniques, can be made very small (<100 square microns), can operate at ambient temperatures, are passive devices (no pumps or valves are needed), and their resistance change can be read-out by a simple, low power (and low current) circuit that measures DC resistance. This feature allows the use of long electrical cables, which permits the resistance measurement unit and data logging equipment to be remotely located on the surface. Also, chemiresistors are resistant to chemical poisoning (unlike catalytic sensors).
A common type of chemiresistor consists of a chemically sensitive, electrically insulating, organic, soluble polymer matrix that is loaded with a large volume (e.g., 20–40%) of electrically conductive metallic (e.g., gold, silver) or carbon particles to form a polymer-particle composite having a network of continuous electrically conductive pathways throughout the polymer matrix (i.e., host). To fabricate a chemiresistor, the polymer is mixed with a solvent (e.g., water, chlorobenzene, or chloroform) and sub-micron diameter carbon, silver, or gold particles (e.g., 20–30 nanometers) to make an “ink”. Then, the resulting ink is deposited onto an insulating substrate as a thin film bridging across two (or more) spaced-apart thin-film electrodes, and then dried. A non-ionic surfactant can also be added to this mixture to chemically bond to the electrically conducting particles and thereby form steric barriers to prevent undesirable aggregation or agglomeration of these particles.
When chemical vapors of solvents, toxic chemicals, explosives, or VOCs come into contact with the polymer-particle composite, the polymer matrix absorbs the vapor(s) and swells. The swelling spreads apart the conductive particles, breaking some of the conductive pathways. This increases the electrical resistance across the two (or more) electrodes by an amount that is easily measured and recorded. The amount of swelling in steady-state, and, hence, the steady-state resistance change, is uniquely related to the concentration of the chemical vapor(s) in equilibrium with the chemiresistor. The resistance response is generally linear with increasing vapor concentration, but can become non-linear at high solvent concentrations when the percolation threshold of the polymer-particle composite is reached. The swelling process is generally reversible; hence the polymer matrix will shrink when the source of chemical vapor is removed (although some hysteresis can occur).
Chemiresistors generally should not be placed in direct contact with liquid VOCs because the polymer can be partially or completely dissolved by the liquid VOC, which may ruin the chemiresistor. Also, it is undesirable to have liquid water in contact with any electrical traces, leads or conductors used in the chemiresistor sensor, because of the potential for problems with short-circuiting and corrosion. Also, direct contact with water can cause the thin-film chemiresistor to detach from the substrate over time. Therefore, for in-situ monitoring of VOCs (e.g., in a well or in water-bearing soil), a waterproof, gas permeable membrane is used to prevent liquid water or liquid VOCs from directly contacting the chemiresistors or exposed electronic components.
The polymer matrix used in chemiresistors generally absorbs multiple solvents having similar solubility parameters. See M. P. Eastman, R. C. Hughes, W. G. Yelton, A. J. Ricco., S. V. Patel and M. W. Jenkins, “Application of the Solubility Parameter Concept to the Design of Chemiresistor Arrays,” Journal of the Electrochemical Society, Vol. 146, pp. 3907–3913, 1999. Since it is unlikely that any one specific polymer will be sensitive to only one particular VOC, an array of multiple chemiresistors containing a variety of polymer hosts is generally needed to provide accurate discrimination among multiple, interfering vapors (including water vapor).
Multiple chemiresistors have been fabricated side-by-side on a common substrate, such as a silicon wafer, where each chemiresistor has a different polymer matrix selected for high sensitivity to a particular VOC of interest. (See R. C. Hughes, et al., “Integrated Chemiresistor Array for Small Sensor Platforms,” SPIE Proceedings, Detection and Remediation Technologies for Mines and Minelike Targets V, Vol 4038-62, pp. 519–529, Apr. 24–28, 2000). The more unknown VOCs there are, the greater the number of different polymers are needed to provide adequate discrimination. Hence, a fast and accurate identification technique is needed that can distinguish between multiple types of solvents (polar and non-polar), for both pure compounds and mixtures, over a wide range of concentrations, and in the presence of water vapor.
A common and obvious source of interfering vapors is water vapor (i.e., relative humidity) in the ambient environment. Water vapor affects the relative sensitivity of certain polymers to solvent vapors, and affects the patterns of responses obtained from arrays containing those polymers. To build a chemiresistor array that is capable of identifying the maximum number of possible analytes, the chemiresistors should be as chemically varied as possible, with at least one chemiresistor having significant sensitivity to water vapor.
Arrays of multiple chemiresistors have been successfully used to detect a wide variety of VOCs, including aromatic hydrocarbons (e.g., benzene), chlorinated solvents (e.g., trichloroethylene (TCE), carbon tetrachloride, aliphatic hydrocarbons (e.g., hexane, iso-octane), alcohols, and ketones (e.g., acetone)). See S. V. Patel, M. W. Jenkins, R. C. Hughes, W. G. Yelton, and A. J. Ricco., “Differentiation of Chemical Components in a Binary Solvent Vapor Mixture Using Carbon/Polymer Composite-Based Chemiresistors,” Analytical Chemistry, Vol. 72, pp. 1532–1542, 2000.
Use of selective gas separation membranes can reduce the maximum number of different chemiresistors in an array since a VOC of particular interest can be selectively passed through the exterior wall of the package housing the chemiresistor array, while excluding other VOCs of lesser interest (e.g. via a selectively permeable membrane). For example, it would be useful to selectively pass chlorinated aliphatic hydrocarbons, but not aromatic hydrocarbons, through the sensor's enclosure.
To miniaturize the electronics that control and drive the chemiresistor arrays Application Specific Integrated Circuits (ASIC) can be integrated with the chemiresistors on a common substrate. The ASIC can perform a variety of functions, including measuring electrical resistance, conditioning data, sensing temperature, and controlling heater elements. (See R. C. Hughes, et al., “Integrated Chemiresistor Array for Small Sensor Platforms,” SPIE Proceedings, Detection and Remediation Technologies for Mines and Minelike Targets V, Vol 4038-62, pp. 519–529, Apr. 24–28, 2000).
Chemiresistor arrays have also been integrated with a thin-film gas preconcentrator module located side-by-side on a common substrate (or facing each other in close proximity). See R. C. Hughes, S. V. Patel, and R. P. Manginell, “A MEMS Based Hybrid Preconcentrator/Chemiresistor Chemical Sensor,” Paper presented at the 198th Meeting of The Electrochemical Society, Phoenix, Ariz., Oct. 22–27, 2000. See also R. C. Hughes, R. P. Manginell, and R. Kottenstette, “Chemical Sensing with an Integrated Preconcentrator/Chemiresistor Array,” Proceedings of Symposium on Chemical and Biological Sensors and Analytical Methods II, Meeting of The Electrochemical Society, San Francisco, Calif., Sep. 2–7, 2001.
The preconcentrator module works by slowly adsorbing VOC vapors into a thin layer of sorbtive material (i.e., “phase”) over a sustained period of time, and then quickly releasing a concentrated puff of the VOC gas by rapidly heating the sorbent using an underlying resistance heater element. The adjacent chemiresistor array is then exposed to a highly concentrated amount of the VOC, which effectively improves the limit of detection of VOCs by factors of 10–1000 times (e.g., from ppm to ppb).
Data analysis techniques for analyzing the resistance response of chemiresistor sensors (or other microchemical sensors) can use the method of Principal Components Analysis (PCA) to identify statistical trends that can distinguish individual VOCs (See, e.g., Lewis and Freund, Sensor Arrays for Detecting Analytes in Fluids, U.S. Pat. No. 5,951,846). However, at high vapor concentrations PCA methods cannot generally be used because the chemiresistor's response becomes non-linear when the percolation threshold is approached.
The non-linear response, however, can be analyzed using pattern recognition techniques (in addition to linear responses). These techniques work by using neural network methods or by comparing the resulting chemical signatures with calibration (i.e., training) sets using advanced pattern recognition software, such as the Visual Empirical Region of Influence (VERI) technique. See G. C. Osbourn, et al., “Visual-Empirical Region-of-Influence Pattern Recognition Applied to Chemical Microsensor Array Selection and Chemical Analysis,” Acc. Chem. Res. Vol. 31, pp. 297–305, 1998. See also U.S. Pat. No. 6,304,675 to Osbourn, et al., “Visual Cluster Analysis and Pattern Recognition Methods”. The “VERI” pattern recognition algorithm performs a similar operation to human vision by analyzing the clustering of the unknown data points with training data from known VOCs.
A desirable aspect of in-situ monitoring is having the capability to not only detect the presence of contaminants, but to also characterize the contaminant in terms of its composition and location of its source. Traditional monitoring techniques require that the monitoring device be in the immediate vicinity of the contaminant to accurately detect and identify the contaminant location. Hence, a need exists for a characterization method that can identify the source's location using one or more in-situ sensors that are located relatively far away from the source.
Current remediation methods use soil vapor extraction (SVE) techniques in the vadose zone (using a vacuum pump to pull vapors out of an extraction (i.e., exhaust) well, or directly coupled to the surface of a shallow spill) and/or air sparging techniques in the saturated zone (pumping air down into a well to below the liquid level to force (i.e., advect) the flow of contaminant vapors through porous soil in the vadose zone towards an exhaust (i.e., exit) well or other opening). These traditional methods rely on the collection of manual samples of the effluent concentration to determine when the site is sufficiently clean. However, heterogeneities in the subsurface can cause rapid decreases in the effluent concentrations caused by mass-transfer limitations, which can be misinterpreted as a “clean” site by a sensor located at the extraction point. The use of an in-situ sensor could not only provide insight into these processes, but it could also be used by automatic feedback systems to turn the pumps on and off when needed, to save money when operating the remediation system.
Existing sensor technologies have not been combined with advanced characterization and automated optimization methods capable of identifying source locations and controlling remediation operations. Single element sensors such as the Adsistor™ are intended to act as an alarm only. Traditional (manual) sampling methods, which are costly and time-consuming, must be subsequently implemented to confirm the veracity of the initial detection and to characterize the contaminant.
A need remains, therefore, for a robust, cost-effective, real-time in-situ microsensor system that can be used for long-term stewardship, and for monitoring subsurface volatile contaminants during remedial operations, such as soil vapor extraction (SVE), air sparging, and bioremediation. Additionally, such a system should also be able to automatically control active components of the remedial system (e.g., air-sparging pumps, vacuum pumps, values, etc.) based on measured in-situ concentrations in order to optimize system performance. A need also exists for wireless communication means that transmit the measured in-situ concentrations (possibly authenticated and encrypted) from a data logger connected to the in-situ sensor to a remote computer for real-time monitoring. Information can also be posted to a Web site for instant dissemination to authorized individuals anywhere in the world.
The ability to perform in-situ diagnostic measurements can save enormous costs associated with traditional manual sampling methods that require off-site analysis. In addition, remediation costs can be reduced by optimally operating components of the remediation system only when needed. Finally, the ability to efficiently conduct remedial measures in real-time (as needed) can improve public confidence in the ability of federal, state, and local governmental agencies to protect the environment and prevent contaminant migration away from these contaminated sites.
Against this background, the present invention was developed.